Embodiments presented herein relate to reverse osmosis elements and more particularly to spiral feed flow reverse osmosis elements.
Reverse osmosis is widely used for purifying fluids such as water. In reverse osmosis, a feed solution such as, brackish or impure water, sea water, and so forth, is passed through a semi-permeable membrane at a pressure higher than the osmotic pressure of the feed water. A permeate, for example, purified water is obtained on the other side of the semi-permeable membrane.
Current reverse osmosis systems typically include cross flow type elements, with feed that flows axially through the element and permeate that flows spirally into the core. Although less common, spiral feed flow elements also exist. Both cross flow elements and spiral feed flow elements include a leaf wound around a core. The leaf may include a layer of permeate carrier sandwiched between two layers of membrane element and a layer of feed spacer, disposed adjacent to one or both membrane element layers. In cross flow elements, the feed solution is fed into the cross flow element axially at high pressure. The feed solution flows through the membrane element, and the permeate flows spirally through the permeate carrier, and into the core. In spiral feed flow elements, the feed solution flows spirally through the element. The permeate is collected in a permeate channel within the core of the spiral feed flow element and discharged at one or both ends of the spiral feed flow element while the retentate is collected in a separate retentate channel within the core, and flows out one or both ends of the spiral feed flow element. The core in a spiral feed flow element includes separate channels for permeate flow and retentate flow.
Usually a number of cross flow elements may be connected in series to achieve high permeate recovery. As permeate is recovered through the cross flow element, the feed velocity decreases in the feed channel. Such a reduction in feed flow velocities may contribute to fouling of the RO membrane surface. One technique for overcoming the reduction in feed flow velocities includes arranging the cross flow elements in a tapered arrangement. The tapered arrangement includes multiple stages plumbed in series. Each stage includes multiple cross flow elements plumbed in parallel. Each successive stage includes fewer cross flow elements in parallel than the preceding stage. For instance, a three stage tapered arrangement may include four cross flow elements in parallel in the first stage, feeding two cross flow elements in parallel in the second stage, which in turn feed a single cross flow element in the third stage. Each stage feeds the retentate to the next stage. However, the tapered arrangements may increase the cost and the complexity of the RO system.
Further, the feed solution pressure may cause the cross flow element to expand and open up the feed channel flow path. Such expansion also decreases the feed velocity. To ensure that the cross flow element does not expand under feed pressure, cross flow elements are typically enclosed in a casing. Also, cross flow elements may undergo telescoping due to the axial load of the feed solution pressure. One solution to prevent telescoping is the use of anti-telescoping devices disposed at the ends of the cross flow elements. However, anti-telescoping devices reduce the active area of the cross flow element, add cost and increase complexity of the RO system.
Spiral feed flow elements have feed channels and permeate channels of approximately equal spiral length. To reduce permeate backpressure to a minimum and achieve high efficiency, a leaf with a short spiral length is required. However, a leaf with a short spiral length results in spiral feed flow elements that have a small exterior diameter, or spiral feed flow elements having a complicated core design to accept multiple short leaves.
Therefore there is a need for a reverse osmosis element that overcomes these and other shortcomings.